Method and apparatus for actuating a subsurface reciprocal well pump

ABSTRACT

A subsurface well pump actuator drive having a pair of slave cylinders and a pair of auxiliary control cylinders mounted on a wellhead and connected to a pumping string of rods for reciprocal pumping movement of a given stroke. A closed accumulator is connected to the slave cylinders and pressurized to such an extent as to liftingly position the string of rods at a balance point in the middle of the pump stroke with automatic valving means supplying hydraulic pressure to the control cylinders to cause upward movement from the midway balance point to the top of the stroke and for supplying hydraulic pressure upon reaching the top of the stroke to cause reverse movement to the bottom of the stroke after reaching the midpoint balance with the slave cylinders providing the majority of work for the upward stroke.

imited States Patent [72] lnventor Jack Tait Lake Dallas, Tex.

[21] App]. No. 873,809

[22] Filed Nov. 4, 1969 [45] Patented Jan. 4, 1972 [73] Assignee Pump Specialties, Inc.

Dallas, Tex.

[54] METHOD AND APPARATUS FOR ACTUATING A SUBSURFACE REClPROCAL WELL PUMP 11 Claims, 4 Drawing Figs.

[52] U.S. Cl 417/390, 60/52 Pl, 417/399 [51 1 Int. Cl F041) 9/08,

F04b 17/00, F15b 15/18 [50] Field of Search 60/52 PJ; 417/398, 399, 390

[5 6] References Cited UNITED STATES PATENTS 1,596,145 8/1926 Black 60/52 PJ 1,619,475 3/1927 Hubbard 60/52 Pl 2,072,595 3/1937 Hutchison 60/52 PJ 2,347,302 4/ 1944 Twyman et al. 60/52 PJ V is 2,504,218 4/1950 Noll et al. 60/52 PJ 2,780,063 2/1957 Bacci 60/52 PJ 2,999,360 9/1961 l-labenicht... 60/52 F] 3,163,005 12/1964 Reed 60/52 PJ Primary Examiner-Carlton R. Croyle Assistant ExaminerJohn J. Vrablik Att0rneys-E. Manning Giles, J. Patrick Cagney, Michael A.

Kondzella and Richard A. Zachar ABSTRACT: A subsurface well pump actuator drive having a pair of slave cylinders and a pair of auxiliary control cylinders mounted on a wellhead and connected to a pumping string of rods for reciprocal pumping movement of a given stroke. A

' closed accumulator is connected to the slave cylinders and i v TH/ZO r725 PATENTEDJAN 4:972 3332.234

SHEET 1 UP 2 THEOTTLE BEL J'AcK Tau-r LAKE muo-gwmltk PATENTED JAN 4572 SHEET 2 BF 2 I: VP. 1 T01: JACK- Tm-r LAKE METHOD AND APPARATUS FOR ACTUATING A SUBSURFACE RECIPROCAL WELL PUMP BACKGROUND OF THE INVENTION Reciprocal pump mounted at the bottom of a well and con nected to the surface by a string of sucker rods which are reciprocated to actuate the pump have long been known and used in the petroleum industry. Normally, the string is lifted to the top of the stroke in a pumping stroke by power means and is gravitationally returned to the bottom of the stroke in order that the cycle can be repeated. A variety of devices have been employed for providing the reciprocal drive to the sucker rod string. However, the prior known drive means for reciprocating the sucker rods have suffered from a number of deficiencies due to their complexity, high cost of manufacture and maintenance and high power requirements.

Probably the most common pump drive previously employed is the well-known rocking arm type pump unit which has many moving parts, bearings, gears, bridles and the like which contribute to its initial high cost and which require constant maintenance. Moreover, units of this type are extremely heavy and cumbersome and require the use of very heavy trucks or other heavy equipment for movement to a pumping installation. Moreover, the maintenance required of units of this type necessitates extremely heavy duty equipment which must be moved to the well site. Additionally, the power requirements for pump units of this type are high and such units are consequently expensive to operate.

Other prior known pump actuators have employed hydraulic means in which a hydraulic cylinder or cylinders provide the means for lifting the sucker rod string during a pumping stroke. However, the prior known devices of this sort have universally required high pressure and power requirements due to the great force that must be exerted on the sucker rod string. The necessity for using high pressure in devices of this type has also necessitated the use of high-quality tubing, valves and the like capable of operating satisfactorily under such high pressure. While a number of the prior known hydraulic devices have employed accumulator means for aiding in their operation, such accumulators have merely provided a counterbalancing force at the bottom of the pumping stroke and it has consequently been necessary to employ power pressure at a very high level for moving from the bottom to the top of the stroke.

One example of prior known pump actuators is found in US. Pat. No. 2,770,197 in which a single cylinder is employed for lifting the sucker rod string with hydraulic pressure being supplied to the cylinder by means of a hydraulic pump having its input connected to a pressure accumulator with a complicated control circuit of valving and hydraulic pump controls necessary for operation of the device. Additionally, the device shown in this patent is somewhat limited as to versatility since a length of stroke can only be adjusted slightly and the device would not be operable with a wide variety of subsurface pumps having differing pump stroke characteristics.

Other prior known hydraulic actuating systems for deep well pumps are found in U.S. Pat. No. 2,564,285 to Smith and US. Pat. No. 2,366,777 issued to Farley et al. The devices of the foregoing patents are illustrative of the cumbersome and complicated nature of the prior known pump actuators which are expensive to manufacture and maintain and are also difficult to set up and maintain. Moreover, many of the prior art pump-driving devices lack versatility and consequently cannot be used with various different down hole pumps having different stroke length, capacities and the like.

SUMMARY OF THE INVENTION It is the primary object of this invention to provide a new and improved pumping unit system that is economical to manufacture, maintain and operate.

Obtainment of the object of this invention is enabled through the provision of a wellhead-mounted pump drive unit comprising a pair of hydraulic slave cylinders and a pair of control cylinders attached to a wellhead and which have their piston rods connected to a common yoke affixed to the top of a sucker rod string. The lower end of the sucker rod string is drivingly connected to the subsurface well pump in a conventional well-known manner. An accumulator is connected to the slave cylinders and is pressurized to a pressure that is sufficiently high so that the pressure in the accumulator will lift the sucker rod string to a balance point in the middle of the pumping unit stroke at which point the string would be maintained by pressure in the accumulator if no additional forces should be applied to the thus-positioned pumping unit. However, a hydraulic pump driven by an electric motor or other source of energy is connected through a control valve to the control cylinders so that pressurized fluid from the hydraulic pump acts on the control cylinders to provide additional force necessary to move the string of rods to the top of the stroke of the pumping unit to complete the pumping stroke of the subsurface pump operation. Upon reaching the top of the stroke, the control valve is automatically reversed to supply pressure to the control cylinders in a reverse manner to urge the sucker rod string downwardly so as to move the string to the bottom of the stroke to terminate the pumping cycle of the subsurface pump. The instant invention is simple in construction and can be carried to the well site in a small vehicle such as a Jeep or station wagon and mounted on the wellhead by a single individual. Moreover, since the majority of the pumping work is done by the slave units, the pressure required for the control cylinder is very low; consequently, a small electric motor or other type prime mover can adequately drive the hydraulic pump employed for activating the control cylinders. The device is easily adjustable in order to vary the length of the power stroke or the number of strokes per minute in accordance with the stroke requirements of the particular subsurface pump being employed in the well. Therefore, the instant invention is uniquely efficient and economical to manufacture and operate and constitutes a great improvement over prior known devices and methods of operations as will become more apparent hereinafter.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a plan view of the preferred embodiment of the invention;

FIG. 2 is a front elevational view of the preferred embodiment with certain parts being shown in section;

FIG. 3 is a sectional view taken along lines 33 of FIG. 2; and

FIG. 4 is a hydraulic circuit diagram of the hydraulic system of the preferred embodiment.

DESCRIPTION OF THE PREFERRED EMBODIMENT Attention is initially invited to FIG. 2 of the drawings which illustrate a conventional wellhead 18 to which the preferred embodiment 20 of the invention is attached for providing reciprocating drive to the topmost polish rod 21 of a string of sucker rods. The bottom rod of the string is connected to a conventional reciprocal piston pump in the well in a wellknown manner.

A baseplate 22 is connected to the top of the wellhead 18 by means of a stand 23 which gives room for tubing collar, pumping tee and stufi'rng box, by bolt and nut assemblies 24. Baseplate 22 provides support for the various components of the preferred embodiment and includes a slot 26 (FIG. 1) extending inwardly from its forward edge and surrounding the sucker rod 21 without contacting the sucker rod so that the sucker rod is free to be reciprocated vertically along its axis; it also provides easy removal of the unit from the wellhead when the well is being worked on, such as when pulling tubing and rods.

First cylinder means in the form of a pair of relatively large slave cylinders 28 are fixedly attached to the main baseplate 22 with one of the cylinders 28 being immediately to the left of sucker rod 21 and the other being immediately to the right of sucker rod 21 as viewed in FIGS. 1 and 2. Pistons 29 in each of the slave cylinders 28 have rods 30 extending upwardly from the slave cylinders and which are connected on their upper ends to a yoke member 32. Yoke member 32 gas the upper end of polish rod 21 connected to its midpoint so that movement of the yoke is conveyed to the sucker rod string. Hydraulic lines 34 extend between the bottom of each of the slave cylinders 28 and the bottom of a pressure accumulator 36 and the upper ends of cylinders 28 are vented to atmosphere so that pistons 29 are free to move to the top of the cylinder 28. Accumulator 36 has a quantity of hydraulic fluid 37 in its lower end with a pressurized nitrogen atmosphere 38 occupying the space above the hydraulic fluid.

Second hydraulic cylinder means in the form of a pair of control cylinders 40 are fixedly attached to baseplate 22 on opposite sides of the slave cylinders 28. Each of the control cylinders 40 has a piston 41 having a rod 42 extending upwardly from its upper end with the rods 42 being fixedly connected to yoke 32 in the same manner as rods 30. The axes of rods 42, rods 30 and sucker rod 21 lie in a common plane and are in parallel alignment as best shown in FIGS. 1 and 2.

A first pressure line 44 is connected to the upper ends of control cylinders 40 and a second pressure line 46 is connected to the lower end of the cylinders. Consequently, pressure introduced through line 46 tends to urge pistons 41 and rods 42 upwardly and pressure introduced through line 44 tends to move rods 42 downwardly. The first pressure line 44 is connected to a first outlet line 48 of a main control valve 50 and the second pressure line 46 is connected to a second outlet line 52 of control valve 50 as best illustrated in FIG. 4.

Control valve 50 includes an internal piston member 54 mounted for reciprocation within the body of the valve between right and left limit positions. A main pump outlet line 56 from a hydraulic pressure pump 58 is connected to the main control valve 50 and an exhaust line 60 is also connected to the control valve. Exhaust line 60 has an open end draining into a sump 62 to which the inlet 64 of pump 58 communicates. A relief valve 66 in the main pump outlet 56 provides pressure relief of any excess pressure in the pump outlet line 56 for safety; but this valve is primarily here to slow the speed of the stroke movement at the upper travel of the stroke and at the lower travel of the stroke; thus a gradual stop of the movement is enabled.

Piston valve 54 is positioned by pressure from a pilot valve 70. Pilot valve 70 has first and second signal lines 72 and 74 respectively and includes an internal piston valve member 76 positionable in either a lower position as illustrated in FIG. 4 or an upper position. A control pressure inlet line 78 is connected to valve 70 from the main pump outlet line 56 and a control valve exhaust line 80 is connected to the main exhaust line 60. Line 80 includes a choke-type throttling valve 82 the purpose of which will be discussed hereinafter.

A movable control rod 84 (FIGS. 2 & 4) extends upwardly from valve 70 and has its lower end connected to the control piston valve member 76 and has an upper lock collar lug 86 fixedly attached adjacent its upper end and a lower lock collar lug 88 attached adjacent its lower end. The position of lugs 86 and 88 on rod 84 can be adjusted to give any desired length of stroke from a few inches to the maximum length of stroke of the unit. A pilot valve actuator trip arm 90 (FIG. 2) is attached to the polish rod 21 for engagement with the lugs 86 and 88 for controlling valve 70. Trip arm 90 engages lug 88 as the sucker rod string approaches the bottom of a pumping stroke as shown in FIG. 2 and engages the upper lug as the sucker rod string approaches the top of a pumping stroke. Engagement of the trip arm 90 with either of lugs 86 or 88 serves to move the rod 84 and the control piston valve member 76 to either an upper or lower position as the case may be. The control piston valve member will remain in either the upper or lower position until subsequent force by the trip arm is applied to move it to the other position.

When control piston valve member 76 is in the lower position illustrated in FIG. 4, it should be noted that control pressure from line 56 flows through valve 70 to signal line 72 to consequently position the piston valve member 54 of the control valve 50 in the right position illustrated in FIG. 4. Positioning of element 54 in the illustrated position causes pressure from line 56 to flow to outlet line 52 and then into the second pressure line 46 which communicates with the lower ends of control cylinders 40. Consequently, yoke 32 is urged upwardly by piston rods 42. However, when the control piston 76 of valve 70 is in its upper position, pressure flows from line 78 to line 74 to consequently move the control piston 54 to a leftward position in which the main pump output is applied to lines 48, 44 so that rods 42 pull downwardly on yoke 32. Simultaneously, line 46 is connected to the exhaust line 60 to permit downward movement of pistons 41.

It should be noted that shifting movement of rod 84 does not result in an immediate shifting of the piston valve member 54 of valve 50 since choke valve 82 regulates the amount of exhaust flow from the low-pressure side of piston 54 following shifting movement of rod 84. Consequently, there is a built-in delay or dwell at the upper and lower limits of the stroke of rod 21 for preventing undesirable rod lash or oscillations. The operation is such that rod 21 comes to a gradual stop in either direction prior to beginning reverse movement in the opposite direction.

In operation, accumulator 36 is initially pressurized to a pressure that is sufficiently high that slave cylinders 28 would move rods 30 etc., upwardly through one-half of the pump stroke to a balance point illustrated in FIG. 4 if no other forces should be exerted on the movable elements supported by yoke 32. The accumulator requires no further pressurization or other attention. When the movable parts are at the balance point, the pressure in accumulator 36 has been reduced to a low level so that the pressure from the accumulator is incapable of moving rod 30 upwardly beyond the balance point and the upward force of slave pistons 29 is exactly equal to the weight on yoke 32, the sucker rod string and the column of oil being lifted. However, control cylinders 40 provide the additional force necessary for enabling obtainment of a complete pump stroke both above and below the balance point.

Assuming that the accumulator pressure has been adjusted to give balance at the pump midstroke and piston rods 30 and 42 are at the bottom of their stroke, no pressure will build up in lines 46 beneath pistons 41 since the slave cylinders are doing all of the lifting of the load as yoke 32 moves upwardly. The yoke will consequently be lifted upwardly and as it moves through the midpoint of the stroke, the force being exerted by slave pistons 29 will be reduced below the level of the gravitational resistance to upward movement; however, then pressure will build up in lines 46 beneath pistons 41 to supply additional necessary force to continue the upward movement of such to the sucker rod string to the top of the stroke. It should be understood that slave cylinders 28 are substantially larger than the control pistons and consequently, the total work performed in the entire upward stroke is largely provided by the slave cylinders 28. Movement of polish rod 21 to the top of its stroke results in engagement of the arm with the upper lug 86 which consequently moves rod 84 upwardly and positions the control piston 76 in its upper position so that a control signal is provided to control line 74 to gradually shift the piston 54 of control valve 50 to its left position. This shifting of the piston valve member 54 to the left causes the pressure from line 56 to be directed to pressure lines 44, 48 and line 46 is simultaneously connected to exhaust line 60. Since there is downward movement caused by gravity there is no pressure on lines 44, 48 until midpoint of balance is reached by the unit; then pressure in lines 44, 48 develops and causes pistons 41 to pull the yoke 32 down to the bottom of the stroke. There will be the same amount of force supplied by the pistons 41 on the downstroke as was supplied by them on the upstroke. A throttle valve 91 has been placed in exhaust line 60 as shown in FIG. 4 to control the exhaust of fluid from valve 54 in order to control the speed of movement from the bottom of the stroke to the midpoint of the stroke on the upstroke and from the top of the stroke to the midpoint of the stroke on the downstroke, with the aid of throttle valve 91 speed of travel of the unit from the top to bottom and from bottom to top of the pumping stroke can be very even though a range of 2 strokes per minute to 16 strokes per minute.

It has been found that the instant invention is capable of operating pumps at a desired rate while employing low-horsepower motors having only a fraction of the horsepower required for previously known pump designs. For example, a S-horsepower motor provides adequate power for pumping a 5,000-foot well employing a pump having a 48-inch stroke operative at 16 strokes per minute. The magnitude of the pumping job done by such a small motor is all the more apparent when one considers that the load that must be lifted and moved 48 inches for each stroke is almost 10,000 pounds.

Additionally, this invention provides great versatility since the pumping stroke can be varied with great accuracy by adjustment of the vertical position of the collar-type lugs 86 and 88 in an obvious manner.

Moreover, the instant invention provides still further economics in that the component parts are largely conventional offthe-shelf items requiring no special machining or fabrication.

I claim:

1. In subsurface well pump apparatus that includes reciprocating driving means for applying lifting and lowering forces to a string of sucker rods that are connected to drive a subsurface pump in an arrangement in which the lifting forces assist in raising a fluid column load that presents maximum loading to said string during upward travel of said string and the lowering forces supplement gravity during downward travel of said string, a slave and accumulator combination for producing full load prestretch of said string and comprising slave hydraulic piston and cylinder means having slave piston means connected to said string and reciprocally movable in unison with said string between a bottom stroke position and a top stroke position, and hydraulic accumulator means connected to said slave hydraulic cylinder means and containing a trapped volume of substantially oil inert gas exerting a pressure of a value to load said slave piston means and said string to an intermediate position to apply a lifting force on said slave piston means at said intermediate position equal to the combined weight of said string and said fluid column load for effecting substantially full load stretch of said string by said accumulator means.

2. In apparatus in accordance with claim 1 and wherein said slave hydraulic piston and cylinder means comprises a pair of individual slave cylinder units disposed symmetrically to and in a common vertical plane with said string, each of said slave cylinder units having an individual piston unit, and a common yoke interconnecting said piston units and said string for joint movement.

3. ln apparatus in accordance with claim 2 and wherein said accumulator means comprises a normally closed tank partially filled with hydraulic fluid and wherein said trapped volume of gas is compressed nitrogen disposed above the hydraulic fluid in said tank.

4. In apparatus in accordance with claim 3 and wherein said reciprocating driving means comprises a hydraulic fluid pressure pump, master hydraulic piston and cylinder means having master piston means connected to said string and reciprocally movable between corresponding bottom and top stroke positions, hydraulic fluid lines and selectively operable valve means for connecting said hydraulic fluid pressure pump to top and bottom extremities of said master hydraulic cylinder means and automatic control means responsive when said apparatus approaches the bottom stroke position for operating said valve means in one mode to supply pressurized hydraulic fluid to the lower extremity of said master hydraulic cylinder means to provide sufiicient force to move said string to said top stroke position and responsive when said apparatus approaches the top stroke position for operating said valve means in a second mode to supply pressurized hydraulic fluid to the up r extremity of said master hydraulic cylinder means to provr e suflicrent force to move said string to said bottom stroke position.

5. ln apparatus in accordance with claim 4 and wherein said automatic control means includes means for providing a dwell of said master hydraulic piston means at the bottom stroke position for reducing rod lash and oscillations of said string.

6. In apparatus in accordance with claim 4 and wherein said automatic control means includes trip means adjustably secured on said string, and an actuator means connected to said valve means and including first limit means responsive to said trip means at the approach to the top stroke position for setting said valve means in said second mode and second limit means responsive to said trip means at the approach to the bottom stroke position for setting said valve means in said one mode.

7. In apparatus in accordance with claim 6 an including means for adjusting the position of said first and second limit means relative to said trip means for adjusting the stroke length between said top and bottom stroke positions.

8. In apparatus in accordance with claim 1 and wherein said reciprocating driving means comprises a hydraulic fluid pressure pump, master hydraulic piston and cylinder means having master piston means connected to said string and reciprocally movable between corresponding bottom and top stroke positions, hydraulic fluid lines and selectively operable valve means for connecting said hydraulic fluid pressure pump to top and bottom extremities of said master hydraulic cylinder means and automatic control means responsive when said apparatus approaches the bottom stroke position for operating said valve means in one mode to supply pressurized hydraulic fluid to the lower extremity of said master hydraulic cylinder means to provide sufi'rcient force to move said string to said top stroke position and responsive when said apparatus approaches the top stroke position for operating said valve means in a second mode to supply pressurized hydraulic fluid to the upper extremity of said master hydraulic cylinder means to provide su fiicient force to move said string to said bottom stroke position.

9. In apparatus in accordance with claim 8 and wherein said automatic control means includes means for providing a dwell of said master hydraulic piston means at the bottom stroke position for reducing rod lash and oscillations of said string.

10. In apparatus in accordance with claim 8 and wherein said automatic control means includes trip means adjustably secured on said string, and an actuator means connected to said valve means and including first limit means responsive to said trip means at the approach to the top stroke position for setting said valve means in said second mode and second limit means responsive to said trip means at the approach to the bottom stroke position for setting said valve means in said one mode.

11. In apparatus in accordance with claim 10 and including means for adjusting the position of said first and second limit means relative to said trip means for adjusting the stroke length between said top and bottom stroke positions. 

1. In subsurface well pump apparatus that includes reciprocating driving means for applying lifting and lowering forces to a string of sucker rods that are connected to drive a subsurface pump in an arrangement in which the lifting forces assist in raising a fluid column load that presents maximum loading to said string during upward travel of said string and the lowering forces supplement gravity during downward travel of said string, a slave and accumulator combination for producing full load prestretch of said string and comprising slave hydraulic piston and cylinder means having slave piston means connected to said string and reciprocally movable in unison with said string between a bottom stroke position and a top stroke position, and hydraulic accumulator means connected to said slave hydraulic cylinder means and containing a trapped volume of substantially oil inert gas exerting a pressure of a value to load said slave piston means and said string to an intermediate position to apply a lifting force on said slave piston means at said intermediate position equal to the combined weight of said string and said fluid column load for effecting substantially full load stretch of said string by said accumulator means.
 2. In apparatus in accordance with claim 1 and wherein said slave hydraulic piston and cylinder means comprises a pair of individual slave cylinder units disposed symmetrically to and in a common vertical plane with said string, each of said slave cylinder units having an individual piston unit, and a common yoke interconnecting said piston units and said string for joint movement.
 3. In apparatus in accordance with claim 2 and wherein said accumulator means comprises a normally closed tank partially filled with hydraulic fluid and wherein said trapped volume of gas is compressed nitrogen disposed above the hydraulic fluid in said tank.
 4. In apparatus in accordance with claim 3 and wherein said reciprocating driving means comprises a hydraulic fluid pressure pump, master hydraulic piston and cylinder means having master piston means connected to said string and reciprocally movable between corresponding bottom and top stroke positions, hydraulic fluid lines and selectively operable valve means for connecting said hydraulic fluid pressure pump to top and bottom extremities of said master hydraulic cylinder means and automatic control means responsive when said apparatus approaches the bottom stroke position for operating said valve means in one mode to supply pressurized hydraulic fluid to the lower extremity of said master hydraulic cylinder means to provide sufficient force to move said string to said top stroke position and responsive when said apparatus approaches the top stroke position for operating said valve means in a second mode to supply pressurized hydraulic fluid to the upper extremity of said master hydraulic cylinder means to provide sufficient force to move said string to said bottom stroke position.
 5. In apparatus in accordance with claim 4 and wherein said automatic control means includes means for providing a dwell of said master hydraulic piston means at the bottom stroke position for reducing rod lash and oscillations of said string.
 6. In apparatus in accordance with claim 4 and wherein said automatic control means includes trip means adjustably secured on said string, and an actuator means connected to said valve means and including first limit means responsive to said trip means at the approach to the top stroke position for setting said valve means in said second mode and second limit means responsive to said trip means at the approach to the bottom stroke position for setting said valve means in said one mode.
 7. In apparatus in accordance with claim 6 and including means for adjusting the position of said first and second limit means relative to said trip means for adjusting the stroke length between said top and bottom stroke positions.
 8. In apparatus in accordance with claim 1 and wherein said reciprocating driving means comprises a hydraulic fluid pressure pump, master hydraulic piston and cylinder means having master piston means connected to said string and reciprocally movable between corresponding bottom and top stroke positions, hydraulic fluid lines and selectively operable valve means for connecting said hydraulic fluid pressure pump to Top and bottom extremities of said master hydraulic cylinder means and automatic control means responsive when said apparatus approaches the bottom stroke position for operating said valve means in one mode to supply pressurized hydraulic fluid to the lower extremity of said master hydraulic cylinder means to provide sufficient force to move said string to said top stroke position and responsive when said apparatus approaches the top stroke position for operating said valve means in a second mode to supply pressurized hydraulic fluid to the upper extremity of said master hydraulic cylinder means to provide sufficient force to move said string to said bottom stroke position.
 9. In apparatus in accordance with claim 8 and wherein said automatic control means includes means for providing a dwell of said master hydraulic piston means at the bottom stroke position for reducing rod lash and oscillations of said string.
 10. In apparatus in accordance with claim 8 and wherein said automatic control means includes trip means adjustably secured on said string, and an actuator means connected to said valve means and including first limit means responsive to said trip means at the approach to the top stroke position for setting said valve means in said second mode and second limit means responsive to said trip means at the approach to the bottom stroke position for setting said valve means in said one mode.
 11. In apparatus in accordance with claim 10 and including means for adjusting the position of said first and second limit means relative to said trip means for adjusting the stroke length between said top and bottom stroke positions. 